In general, a wire electric discharge machining apparatus is equipped with an automatic wire threader which is capable of automatically threading a wire electrode through a start hole. A wire electrode of 0.1 mm to 0.3 mm in diameter is typically used. The start hole is formed in the work in advance to wire electric discharge machining. If the wire electrode is broken during the machining process, the automatic wire threader threads the wire electrode through a machined kerf in order to commence machining again. A guide pipe for guiding the wire electrode straight to the start hole or the machined kerf is provided to many of automatic wire threaders. The start hole and the machined kerf are hereinafter collectively referred to as the start hole.
The gap (hereinafter referred to as a “clearance”) between the wire electrode and the start hole is small. Therefore, failure may occur if the front end of the wire electrode does not reach the start hole or is caught by the inner wall of the start hole. The feeding of the wire electrode is halted slightly after such failure is detected. At the time, the wire electrode is already loosened. The wire is tightened after the wire electrode is winded. Then, the wire electrode is again sent towards to the start hole. The failure in an automatic wire threading lengthens the required operation time.
The patent document 1 disclosed a method for eliminating the loosening of a wire electrode to avoid a repetitive failure in an automatic wire threading operation. The patent document 2 disclosed a method for guiding a wire electrode to the start hole by applying horizontal micro-vibration with compressed air. The patent document 3 disclosed a method for searching a start hole automatically and positioning the front end of the wire electrode to the entrance of a start hole.
The patent document 4 disclosed a device that eliminates a stuck wire electrode by moving the wire electrode up and down slightly. According to the invention, the operation of the device is similar to an action that a person pinches a thread and tries to thread its front end though a small hole again and again. The wire electrode is threaded through the start hole without being winded, and the time required for automatic wire threading is shortened. It has been proven that the probability of the wire electrode threading through the start hole is high even when the upper wire guide is away from the start hole.